High dielectric constant type ceramic composition

ABSTRACT

There is disclosed a high dielectric constant material which comprises a composite material comprising at least one of BaO and CaO; PbO; Fe 2  O 3  ; Nb 2  O 5  ; WO 3  and CuO, said composite material being represented by the general formula: 
     
         xPb(Fe.sub.1/2 Nb.sub.1/2 l )O.sub.3 -yM(Cu.sub.1/2 W.sub.1/2 )O.sub.3 
    
      -zPb(Fe 2/3  W 1/3  )O 3   
     wherein M is at least one of Ba and Ca, and x, y and z are defined in the specification. 
     The disclosed dielectric material can also contain 0 to 1.0% by weight, relative to said composite material, of manganese oxide.

BACKGROUND OF THE INVENTION

This invention relates to high dielectric constant type ceramiccompositions, more particularly to low-temperature sintering typeceramic compositions which are suitable for multilayer capacitors havinga high dielectric constant and which can be synthesized by a solid phasereaction of a composite material of oxides.

As compositions having a high dielectric constant, composite dielectricceramics have heretofore widely been used in which barium titanate(BaTiO₃) is included as a main component, and calcium zirconate (CaZrO₃)and calcium titanate (CaTiO₃) are further present.

These ceramics must be sintered usually at a temperature as high as1200° to 1400° C., so that for multilayer capacitors including theseceramics, it is disadvantageously required to employ internal electrodeseach comprising mainly an expensive noble metal such as gold, platinumor palladium which can withstand co-firing at such a high temperature.

Further, in sintering these ceramics, a great deal of heat energy ofelectric power, a gas or the like is necessary because of a treatment atelevated temperatures, and in consequence a used sintering furnace,sintering crucibles or the like will thus be prematurely deteriorateddue to the heat, which fact disadvantageously becomes one cause for theincrease in costs for the ceramic capacitors.

For this reason, there has been desired a development of compositionswhich permit employing inexpensive internal electrodes each composed ofsliver or the like as a main component; accomplishing the sinteringoperation even at a temperature as low as 1000° C. or less; andproviding the ceramics having a large dielectric constant and a smalldielectric loss.

Materials comprising Pb(Fe_(2/3) W_(1/3))_(x) (Fe_(1/2) Nb_(1/2))_(1-x)O₃ (wherein x is in the relation of 0.2≦x≦0.5) are known which have beendeveloped in reply to the aforesaid desire and are described in JapaneseProvisional Patent Publication No. 87700/1977 (which corresonds to U.S.Pat. No. 4,078,938). Further, an attempt to add SiO₂ to the aforesaidmaterials has been made in Japanese Provisional Patent Publication No.15591/1978. These mateials are, however, large in temperature dependenceof the dielectric loss (tan δ), and hence, when the multilayercapacitors are prepared by the use of such materials, they will beunpracticable.

Moreover, the multilayer capacitors which have been manufactured fromthe aforesaid materials are weak particularly in mechanical strength andthus have the serious practical problem that cracks and breakage areliable to occur.

An object of this invention is accordingly to provide low-temperaturesintering type ceramic compositions having a high dielectric constant bywhich the aforementioned drawbacks are eliminated and which are high indielectric constant, are excellent in temperature dependence of tan δ,mechanical strength and bias dependence of the dielectric constant, andare improved in high-temperature load properties and moistureresistance.

SUMMARY OF THE INVENTION

The high dielectric constant type ceramic compositions according to thisinvention are each composed of at least one of barium oxide and calciumoxide; lead oxide; iron oxide; niobium oxide; tungsten oxide; and copperoxide, and it is characterized in that when the ceramic composition isrepresented by the general formula:

    xPb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -yM(Cu.sub.1/2 W.sub.1/2)O.sub.3 -zPb(Fe.sub.2/3 W.sub.1/3)O.sub.3

wherein M is at least one of Ba and Ca,

and when A, B, C and D are defined as follows:

    ______________________________________                                        A:       x = 65,    y = 5,     z = 30,                                        B:       x = 90,    y = 10,    z = 0,                                         C:       x = 65,    y = 0.5,   z = 34.5, and                                  D:       x = 99.5,  y = 0.5,   z = 0;                                         ______________________________________                                    

and 0 to 1.0% by weight of manganese oxide (MnO) with respect to theceramic composition having a high dielectric constant within acompositional range surrounded by the above defined A, B, C and D isfurther included in the ceramic composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three-component composition diagram illustrating acompositional range of ceramic compositions according to this invention;

FIGS. 2 to 5 show curves illustrating properties of the high dielectricconstant type ceramic compositions according to this invention;

FIGS. 6 to 9 likewise show curves illustrating properties of ceramiccomposition having a high dielectric constant obtained in ReferenceExamples; and

FIG. 10 shows curves illustrating bias dependences dielectric constants.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The reason why components employed in this invention are limited asfollows.

First, when the ceramic compositions according to this invention arerepresented by the formula;

    xPb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -yM(Cu.sub.1/2 W.sub.1/2)O.sub.3 -zPb(Fe.sub.2/3 W.sub.1/3)O.sub.3

wherein M is at least one of Ba and Ca,

the components of the ceramic composition are limited to the rangesurrounded by the following A, B, C and D:

    ______________________________________                                        A:       x = 65,    y = 5,     z = 30,                                        B:       x = 90,    y = 10,    z = 0,                                         C:       x = 65,    y = 0.5,   z = 34.5, and                                  D:       x = 99.5,  y = 0.5,   z = 0;                                         ______________________________________                                    

because when x is less than 65, the dielectric constant at roomtemperature will be lowered, so that the resulting composition will notbe eligible as the material for the capacitor; when y is less than 0.5,the effects of M(Cu_(1/2) W_(1/2))O₃ (M: Ba and/or Ca), which can lowersintering temperature and the tan δ and can improve DC bias propertiesof the dielectric constant and enhance mechanical strength, will bealmost negligible; and when y exceeds 10, the tan δ will increase.

The compound MnO to be used in this invention serves to improvetemperature dependence of the tan δ of the ceramic compositions andhigh-temperature load properties as well as moisture and temperatureresistances of the manufactured multilayer chip capacitors, and theamount of the additive MnO is within the range of 0 to 1.0% by weight,preferably 0.01 to 1.0% by weight. The reason why the amount of the MnOis restricted to 1.0% by weight or less is that when its amount is inexcess of 1.0% by weight, the tan δ of the obtained ceramics willincrease.

Now, FIG. 1 shows a three-component composition diagram of theaforementioned composition (exclusive of MnO).

The high dielectric constant type ceramic compositions according to thisinvention may be manufactured, for example, as follows:

At a first step, starting materials of lead oxide (PbO), iron oxide (Fe₂O₃), tungsten oxide (WO₃), niobium oxide (Nb₂ O₅), calcium carbonate(CaCO₃), barium carbonate (BaCO₃), copper oxide (CuO), manganesecarbonate (MnCO₃) and the like are employed, and they are weighed in aformulating proportion of this invention. These materials are mixedunder a wet condition by means of a ball mill, followed by calcining ata temperature of 700° to 800° C. Next, a milling operation is carriedout by the ball mill again, and a binder such as a polyvinyl alcohol(PVA) or the like is then added to the milled and dried powder, followedby preparing disk-like bodies of 16.0 mm in diameter and 1.2 mm inthickness under a pressure of approximately 0.8 ton/cm². The thus moldeddisks are placed in a crucible made of magnesia, and a sintering is thenaccomplished at a temperature of 850° to 950° C. for a period of 2hours. Further, a silver paste is printed on the resulting sinters at atemperature of 600° to 700° C. in order to obtain the desired ceramiccompositions according to this invention.

The compositions according to this invention can be applied to theceramic multilayer capacitors, for example, in the following manner.

First, the compositional materials used in the present invention aremixed under a wet condition, and after sintering, milling and dryinghave been carried out to prepare a powder, a binder such as polyvinylbutyral, polyethylene glycol or octyl phthalate and a solvent such astrichloroethylene or ethyl alcohol are suitably added to the powder inorder to prepare a slurry. Then, the slurry is formed into sheets ofapproximately 50 μm in thickness by the use of a doctor blade, andelectrodes are printed on the sheets, which are then laminated. Theresulting laminated sheet material is cut into several chips andintegrally sintered, and termination electrodes are attached to theresulting chips, thereby obtaining the desired ceramic multilayercapacitors.

This invention will be further described in detail in reference toExamples below, but it is not to be intended that any restriction ofthis invention is made by them.

EXPERIMENT 1

Lead oxide (PbO), iron oxide (Fe₂ O₃), tungsten oxide (WO₃), niobiumoxide (Nb₂ O₅), calcium carbonate (CaCO₃), barium carbonate (BaCO₃),copper oxide (CuO) and manganese carbonate (MnCO₃) were used as startingmaterials, and they were weighed in formulating proportions shown inTable 1. The materials for each sample were mixed under a wet conditionby means of a ball mill, followed by calcining at 700° to 800° C.Subsequently, the resulting powder is milled by the ball mill again anddried to prepare a powder. A polyvinyl alcohol as a binder was added tothe powder, and after a mixing operation, forming was carried out undera pressure of approximately 0.8 ton/cm², thereby obtaining disk-likemolded specimens each having a diameter of 16.0 mm and a thickness of1.2 mm. These formed specimens were placed in a crucible made ofmagnesia, and a sintering was accomplished at a temperature of 850° to950° C. for a period of 2 hours, as set forth in Table 1. A silver pastewas printed on the resulting disks to obtain 68 samples (Examples 1 to68).

For these samples, dielectric constant, dielectric loss (tan δ) andresistivity were measured. Results obtained are all set forth inTable 1. The measurement of the aforesaid dielectric constant andsielectric loss were carried out at a frequency of 1 KHz.

COMPARATIVE EXPERIMENT

The same procedure as in Experiment 1 was repeated with the exceptionthat proportions of the starting materials were varied as in ReferenceExamples of Table 1, in order to prepare 12 samples (Reference Examples1 to 12).

For these samples, dielectric constant, tan δ and resistivity weremeasured in the same manner as in Experiment 1. Results obtained are setforth together in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                         Sintering     Resisti-                            Pb(Fe1/2Nb1/2)O.sub.3                                                                 Pb(Fe2/3W1/3)O.sub.3                                                                  M(Cu1/2W1/2)O.sub.3                                                                   MnO Tempera-                                                                           Dielectric                                                                          tan δ                                                                      vity                                x(mole %)                                                                             z(mole %)                                                                             y(mole %)                                                                             wt %                                                                              ture (°C.)                                                                  constant                                                                            (%)                                                                              (Ω.cm)               __________________________________________________________________________    Example                                                                              1   99.5  --      M = Ca                                                                             0.5                                                                              --  920  11300 2.1                                                                              2 × 10.sup.11        "      2 90      --      "    10 --  860  12500 2.9                                                                              3 × 10.sup.11        "      3 95       1      "    4  --  880  21300 1.6                                                                              2 × 10.sup.11        "      4 90       5      "    5  --  880  26000 0.7                                                                              5 × 10.sup.11        "      5 90       7      "    3  --  900  21000 2.2                                                                              2 × 10.sup.11        "      6 85      10      "    5  --  880  20300 0.9                                                                              5 × 10.sup.11        "      7 85      12      "    3  --  900  29200 0.4                                                                              4 × 10.sup.11        "      8 85      14      "    1  --  920  19600 1.5                                                                              3 × 10.sup.10        "      9 80      19      "    1  --  920  20000 0.9                                                                              6 × 10.sup.10        "     10 80      18      "    2  --  900  23000 0.5                                                                              2 × 10.sup.11        "     11 80      16      "    4  --  880  14200 0.9                                                                              5 × 10.sup.11        "     12 80      14      "    6  --  880  10200 1.2                                                                              5 × 10.sup.11        "     13 75      24      "    1  --  920  15300 0.8                                                                              5 × 10.sup.10        "     14 75      22      "    3  --  900  26200 0.8                                                                              6 × 10.sup.11        "     15 70      28      "    2  --  900  19900 0.6                                                                              9 × 10.sup.10        "     16 65        34.5  "    0.5                                                                              --  920  26600 1.4                                                                              4 × 10.sup.10        "     17 65      30      "    5  --  880  14800 1.0                                                                              1 × 10.sup.11        Reference                                                                              80      20           0  --  950   8400 5.3                                                                              8 × 10.sup.7         Example                                                                              1                                                                      Reference                                                                              70      30           0  --  950  20300 4.6                                                                              6 × 10.sup.7         Example                                                                              2                                                                      Reference                                                                              60      40           0  --  950  19500 3.7                                                                              5 × 10.sup.7         Example                                                                              3                                                                      Example                                                                             18 95      --      M = Ca                                                                             5  0.05                                                                              900  22000 0.4                                                                              4 × 10.sup.11        "     19 90      --      "    10 1.0 860  12000 2.2                                                                              6 × 10.sup.11        "     20 95       1      "    4  0.5 880  22000 1.5                                                                              6 × 10.sup.11        "     21 90       5      "    5  0.8 880  25800 0.7                                                                              5 × 10.sup.11        "     22 90       7      "    3  0.5 900  22000 1.9                                                                              4 × 10.sup.11        "     23 85      10      "    5  0.5 880  21600 0.8                                                                              1 × 10.sup.12        "     24 85      12      "    3  0.25                                                                              900  32500 0.25                                                                             9 × 10.sup.11        "     25 85      14      "    1  0.25                                                                              920  20000 1.0                                                                              2 × 10.sup.11        "     26 80      19      "    1  0.25                                                                              920  20500 0.4                                                                              2 × 10.sup.11        "     27 80      18      "    2  0.25                                                                              900  23500 0.5                                                                              5 × 10.sup.11        "     28 80      16      "    4  0.25                                                                              880  14000 0.7                                                                              7 × 10.sup.11        "     29 80      14      "    6  0.1 880  10600 1.0                                                                              8 × 10.sup.11        "     30 75      24      "    1  0.1 920  16200 0.7                                                                              2 × 10.sup.11        "     31 75      22      "    3  0.1 900  27000 0.7                                                                              9 × 10.sup.11        "     32 70      28      "    2  0.1 900  19400 0.4                                                                              2 × 10.sup.11        "     33 65        34.5  "    0.5                                                                              0.5 920  25000 1.1                                                                              2 × 10.sup.11        "     34 65      30      "    5  0.5 880  15000 0.8                                                                              5 × 10.sup.11        Reference                                                                              80      20           0  0   950   8400 5.3                                                                              8 × 10.sup.7         Example                                                                              4                                                                      Reference                                                                              65      12      M = Ca                                                                             3  0   900  29200 0.4                                                                              4 × 10.sup.11        Example                                                                              5                                                                      Reference                                                                              60      40           0  2.0 950  18000 6.3                                                                              2 × 10.sup.6         Example                                                                              6                                                                      Example                                                                             35 95      --      M = Ba                                                                             5  --  900  19000 0.8                                                                              3 × 10.sup.11        "     36 90      --      "    10 --  860  16500 2.6                                                                              2 × 10.sup.11        "     37 95       1      "    4  --  880  22300 2.2                                                                              4 × 10.sup.11        "     38 90       5      "    5  --  880  19500 1.6                                                                              7 × 10.sup.11        "     39 90       7      "    3  --  900  25300 2.0                                                                              5 × 10.sup.11        "     40 85      10      "    5  --  880  29000 0.7                                                                              4 × 10.sup.11        "     41 85      12      "    3  --  900  26500 0.95                                                                             2 × 10.sup.11        "     42 85      14      "    1  --  920  21000 1.6                                                                              4 × 10.sup.10        "     43 80      19      "    1  --  920  20000 1.0                                                                              5 × 10.sup.10        "     44 80      18      "    2  --  900  33000 0.3                                                                              6 × 10.sup.10        "     45 80      16      "    4  --  880  22000 0.9                                                                              7 × 10.sup.11        "     46 80      14      "    6  --  880  11600 0.8                                                                              8 × 10.sup.11        "     47 75      24      "    1  --  920  17800 0.9                                                                              3 × 10.sup.11        "     48 75      22      "    3  --  900  29200 0.6                                                                              6 × 10.sup.10        "     49 70      28      "    2  --  900  22000 0.8                                                                              5 × 10.sup.11        "     50 65        34.5  "    0.5                                                                              --  920  28000 1.6                                                                              5 × 10.sup.11        "     51 65      30      "    5  --  880  19900 0.9                                                                              2 × 10.sup.10        Reference                                                                              80      20           0  --  950   8400 5.3                                                                              8 × 10.sup.7         Example                                                                              7                                                                      Reference                                                                              70      30           0  --  950  20300 4.6                                                                              6 × 10.sup.7         Example                                                                              8                                                                      Reference                                                                              60      40           0  --  950  19500 3.7                                                                              5 × 10.sup.7         Example                                                                              9                                                                      Example                                                                             52 95      --      M = Ba                                                                             5  0.05                                                                              900  19200 0.7                                                                              4 × 10.sup.11        "     53 90      --      "    10 1.0 860  15500 2.5                                                                              3 × 10.sup.11        "     54 95       1      "    4  0.5 880  24000 2.0                                                                              6 × 10.sup.11        "     55 90       5      "    5  0.8 880  19000 1.5                                                                              7 × 10.sup.11        "     56 90       7      "    3  0.5 900  26200 1.5                                                                              8 × 10.sup.11        "     57 85      10      "    5  0.5 880  29500 0.5                                                                              8 × 10.sup.11        "     58 85      12      "    3  0.25                                                                              900  25200 0.6                                                                              4 × 10.sup.11        "     59 85      14      "    1  0.25                                                                              920  22000 1.1                                                                              5 × 10.sup.11        "     60 80      19      "    1  0.25                                                                              920  20500 0.9                                                                              1 × 10.sup.11        "     61 80      18      "    2  0.25                                                                              900  34000 0.2                                                                              1 × 10.sup.12        "     62 80      16      "    4  0.25                                                                              880  22400 0.5                                                                              9 × 10.sup.11        "     63 80      14      "    6  0.1 880  12900 0.7                                                                              1 × 10.sup.12        "     64 75      24      "    1  0.1 920  15000 0.6                                                                              2 × 10.sup. 11       "     65 75      22      "    3  0.1 900  30000 0.5                                                                              7 × 10.sup.11        "     66 70      28      "    2  0.1 900  23300 0.5                                                                              6 × 10.sup.11        "     67 65        34.5  "    0.5                                                                              0.5 920  28500 1.3                                                                              1 × 10.sup.11        "     68 65      30      "    5  0.5 880  19900 0.7                                                                              3 × 10.sup.11        Reference                                                                              80      20           0  0   950   8400 5.3                                                                              8 × 10.sup.7         Example                                                                             10                                                                      Reference                                                                              80      18      M = Ba                                                                             2.0                                                                              0   950  33000 0.4                                                                              6 × 10.sup.11        Example                                                                             11                                                                      Reference                                                                              60      40           0  2.0 950  18000 6.3                                                                              2 × 10.sup.6         Example                                                                             12                                                                      __________________________________________________________________________

Reference Examples in Table 1 are outside the range of this inventionand are included for comparison.

As shown in Table 1, the sample within the range of this inventionexhibit as high dielectric constants as 29,200 to 34,000 in some cases,and permit the sintering temperature of not more than 900° C. under asextremely small dielectric losses as 0.2 to 0.4%.

Further, FIGS. 2, 3, 4 and 5 exhibit the temperature dependence of thedielectric constant and the tan δ in Examples 7, 24, 44 and 61,respectively. In addition, FIGS. 6, 7, 8 and 9 exhibit the temperaturedependence of the dielectric constant and the tan δ in ReferenceExamples 1, 5, 7 and 11, respectively, for reference.

As is apparent from Table 1 and FIGS. 2 to 9, in the cases of thematerials each including the compound M(Cu_(1/2) W_(1/2))O₃ in the formof a solid solution according to this invention, the dielectric constantis outstandingly improved and is 3.5 to 4 times larger than aconventional one. Moreover, it is clear that the dielectric loss tan δis noticeably lowered up to 1/10 or less of a conventional level.

The addition of a small amount of MnO permits the tan δ at a lowtemperature (-55° C.) to be remarkably lowered up to approximately 1/2to 1/5 of a conventional level, and it is to be noted that thedielectric loss tan δ at a high temperature of 125° C. is also reducedup to approximately 2/3 to 1/2 of a conventional level.

EXPERIMENT 2

With regard to the samples of Examples 18 and 58, among the samplesprepared in Experiment 1, as well as Reference Example 2, biasdependence of the dielectric constant was examined. The samples to beexamined were formed to a thickness of 0.2 mm, and silver electrodeswere then attached thereto, they being 13 mm in diameter. Resultsobtained are presented in FIG. 10. In this drawing, curves a, b and ccorrespond to Example 18, Example 58 and Reference Example 2,respectively.

As be apparent from FIG. 10, the three samples are substantially almostsimilar in the dielectric constant, but it has been confirmed that thesamples including Ca(Cu_(1/2) W_(1/2))O₃ and Ba(Cu_(1/2) W_(1/2))O₃ inamounts of 5 mole % and 3 mole %, respectively, are small in the biasdependence or the dielectric constant, which fact is practicallyadvantageous.

EXPERIMENT 3

With regard to some of the samples prepared in Experiment 1, mechanicalstrength was examined, together with samples of several ReferenceExamples.

For the estimate of the mechanical strength, a bending strength wasemployed. The measurement of the bending strength was carried out asfollows.

Each dielectric ceramic disk obtained was lapped on both the surfacesthereof up to a thickness of 1 mm in order to finish it in the state ofa mirror surface. Afterward, by means of a diamond cutter, a samplepiece having a width of 3 mm was cut off from the central portion ofeach disk, and exposed cut surfaces thereof were abraded with SiC sandpapers in order of #800, #1500 and #2000 thereof. Finishing was thencarried out by rounding edges of the plates, and a three-point bendingtest was carried out by the use of an Instron type universal tester.

The bending strength (deflective strength) can be obtained by thefollowing formula:

    Deflective strength=(3/2)·(Pml/d.sup.2 w)

wherein

Pm is a maximum breaking load (kg);

l is a distance (cm) between supporting points;

d is a thickness (cm) of a sample; and

W is a width (cm) of the sample.

Data of the Examples regarding this invention are set forth in Table 2together with those of Reference Examples.

                  TABLE 2                                                         ______________________________________                                                       Bending strength                                                              [kg/cm.sup.2 ]                                                 ______________________________________                                        Example - 7      870                                                          Example - 10     820                                                          Reference Example - 1                                                                          590                                                          Example - 24     890                                                          Example - 27     850                                                          Reference Example - 4                                                                          590                                                          Example - 40     880                                                          Example - 44     790                                                          Reference Example - 7                                                                          590                                                          Example - 57     900                                                          Example - 61     800                                                          Reference Example - 10                                                                         560                                                          ______________________________________                                    

The results in Table 2 definitely indicate that the addition of a smallamount of M(Cu_(1/2) W_(1/2))O₃ (M: Ba and/or Ca) in the form of a solidsolution also permits a remarkable improvement in the mechanicalstrength.

EXPERIMENT 4

Multilayer capacitors were prepared from some samples obtained inExperiment 1, and a high-temperature load test and a moisture-resistanttest were carried out.

These capacitors to be used for these tests were obtained as follows.

To each sintered powder were added suitable amounts of a binder selectedfrom polyvinyl butyral, polyethylene glycol or octyl phthalate and asolvent such as trichloroethylene or ethyl alcohol in order to prepare ausual slurry. Afterward, a doctor blade was utilized to form the slurryinto sheets of 50 μm in thickness, and electrodes were then printed onthe sheets. A plurality of the printed sheets was laminated, cut andsintered. Finally, termination electrodes were attached to the sinteredchips in order to prepare multilayer ceramic capacitors each having 4.5mm×3.2 mm and 1 μF. One hundred multilayer capacitors were subjected tothe high-temperature load test and the moisture-resistant test which aredescribed in the paragraph of Multilayer Capacitors (Chip-form) forElectric Eqipments of Nippon Denki Kogyokai Kikaku (Japanese ElectricMachine Industrial Institute Standards) RC-3698B, and failure rate andchange in volume were examined after thses tests. The results obtainedare set forth in Tables 3 and 4, respectively.

In this case, the tests above were accomplished in accordance with theaforesaid standards, i.e., in the instance where the high-temperatureload test is concerned, a state due to the application of a 50 V DC andpropoerties after 1000 hours at 125° C. were measured; in the instancewhere the moisture-resistant test is concerned, a state due to theapplication of a 25 V DC and properties after 500 hours at 40° C. and at95% RH were measured.

                  TABLE 3                                                         ______________________________________                                                  Failure Rate                                                                           Rate of Change                                                       (%)      in Capacitance (%)                                         ______________________________________                                        Example 24  0          -5                                                     Example 27  0          -2                                                     Reference   3          -15                                                    Example 4                                                                     Reference   1          -11                                                    Example 5                                                                     Example 57  0          -4                                                     Example 61  1          -3                                                     Reference   3          -12                                                    Example 10                                                                    Reference   2          -11                                                    Example 11                                                                    ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                  Failure Rate                                                                           Rate of Change                                                       (%)      in Capacitance (%)                                         ______________________________________                                        Example 24  0          -4                                                     Example 27  1          -5                                                     Reference   6          -13                                                    Example 4                                                                     Reference   3          -10                                                    Example 5                                                                     Example 57  0          -6                                                     Example 61  0          -5                                                     Reference   5          -13                                                    Example 10                                                                    Reference   3          -11                                                    Example 11                                                                    ______________________________________                                    

As is definite from Tables 3 and 4, it has been confirmed that when asmall amount of MnO is added to the composition, the capacitor made fromthis composition will noticeably improve in the failure rate, and willbe reduced in the rate of the change in capacitance, and will beexcellent in the high-temperature load properties and the moistureresistance.

Furthermore, by using Pb(Fe_(1/2) Nb_(1/2))O₃ -M(Cu_(1/2) W_(1/2))O₃ (Mrepresents Ba or Ca) binary system material composition, a mechanicalstrength thereof was measured. As the result, excellent mechanicalstrength was obtained.

Namely, the mechanical bending strength of each Examples 52 and 53 wasmesured. Measurement method thereof is the same as mentioned above andthe results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                   Bending strength                                                              [kg/cm.sup.2 ]                                                     ______________________________________                                        Example 52   1050                                                             Example 53   1120                                                             ______________________________________                                    

As is apparent from the Table, Pb(Fe_(1/2) Nb_(1/2))O₃ -M(Cu_(1/2)-W_(1/2))O₃ (M represents Ba or Ca) binary system show more than 1000kg/cm² of the bending strength which is the value almost comparative tothat of BaTiO₃ ceramic which is mainly used as a dielectric for themultilayer ceramic capacitor.

Although oxides and carbonates were used as starting materials in theaforesaid Examples, it is a matter of course that similar effects can beobtained likewise by using organic metallic compounds such as oxalates,in place of the oxides, at a high temperature.

As is apparent from the foregoing, according to this invention,low-temperature sintering type ceramic composition having a highdielectric constant can be obtained which are high in the dielectricconstant, are excellent in the temperature properties of the tan δ, thebias dependence of the dielectric constant and the mechanical strength,and are improved in the high-temperature load properties and themoisture resistance. Hence, the compositions according to this inventionare considered to be extremely excellent on an industrial application.

We claim:
 1. A ceramic composition having a high dielectric constant consisting essentially of lead oxide, iron oxide, niobium oxide, tungsten oxide, copper oxide, and at least one of barium oxide and calcium oxide, said ceramic composition being represented by the formula:

    xPb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -yM(Cu.sub.1/2 W.sub.1/2)O.sub.3 -zPb(Fe.sub.2/3 W.sub.1/3)O.sub.3

wherein M is at least one of Ba and Ca, and wherein x, y and z are defined as follows:

    ______________________________________                                         [A:]      x = 65,    y = 5,     z = 30, or                                     [B:]      x = 90,    y = 10,    z = 0, or                                      [C:]      x = 65,    y = 0.5,   z = 34.5, or                                   [D:]      x = 99.5,  y = 0.5,   z = 0                                          ______________________________________                                    


2. A ceramic composition according to claim 1, wherein said ceramic composition is represented by the formula:

    xPb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -yM(Cu.sub.1/2 W.sub.1/2)O.sub.3

wherein x and y satisfy the relations of 90≦x≦99.5 and 0.5≦y≦10, respectively.
 3. A ceramic composition according to claim 2, further consisting essentially of 1.0% by weight or less, with respect to said ceramic composition, of manganese oxide.
 4. A ceramic composition according to claim 1, further consisting essentially of 1.0% by weight or less, with respect to said ceramic composition, of manganese oxide. 